Natural gas occurs naturally in underground fossil fuel deposits or formations. Some formations contain relatively few hydrocarbons that are liquid at ambient temperatures. When produced, such as via a drilled well, these formations produce natural gas and are termed “gas wells,” in contrast to wells that produce primarily liquid hydrocarbons. As-produced, natural gas is typically a mixture of methane (single carbon, formula CH4) with varying concentrations of other gases, which may include C2+ hydrocarbons, carbon dioxide, and inert gases such as nitrogen. By way of example, Table 1 below gives exemplary ranges for the proportion of several components that may be present in produced natural gas. Table 1 also includes the proportion of each component that typically must be present in commercial grade gas, i.e. gas that is worth processing and shipping. Notably, nitrogen and carbon dioxide can each occur naturally at levels well above the commercially acceptable range. In such cases, it is necessary to treat the produced natural gas.
TABLE 1Naturally OccurringCommercial RangeComponentRange (mole %)(mole %)Methane25-100>70Ethane0-200-20PropaneButane (iso-, normal-)Pentane (iso-, normal-)trace-0.14 —Hexanes plustrace-0.06 —Nitrogen0-50<4Carbon Dioxide0-60<1.0Oxygen 0-0.2<0.1Hydrogen<0.02Hydrogen sulphide0-5 —Rare gases (A, He, Ne, Xe)trace—
Whether a given gas well, non conventional biogas generator, or syngas facility is worth producing or operating depends on the relative amounts of the hydrocarbons, which are valuable as fuels, and other gases, which have little or no value. Nitrogen and carbon dioxide are inert gases with no BTU value. If low-value gases are present at high levels in a produced gas stream, their concentration must be reduced to low levels (typically <4% for nitrogen) before the gas can be sold. At present, many gas wells are shut in, i.e. capped and non-producing, because the mixture of gases they produce contains too few hydrocarbons to justify the cost of production and separation. It has been estimated that much as 15% of the United States' natural gas reserves contain too much nitrogen to be shipped as-is.
U.S Pat. No. 6,444,012 provides a good description of various methods that have been used to remove or reduce the concentration of nitrogen in natural gas. Despite the advances that have been made in this area, however, current technologies for separating the hydrocarbons from the other gases remain less than satisfactory. Hence, it is desirable to provide a system in which methane and C2+ hydrocarbons can be inexpensively and effectively separated from other gases that may be present in a produced natural gas stream.